Pilot project to investigate the effect age has on homeostatic responses to nutritional manipulation. Six healthy men and women aged 20-39 and twelve healthy subjects 60+ will be studied after a 12-hour overnight fast (Baseline) and at 3 occasions at least one week apart: following the oral ingestion of an 800 calorie source of carbohydrate, protein, and mixed nutrient (45% carbohydrate, 40% fat, 15% protein). A sub-group of six healthy subjects from both age groups will be studied at baseline and following a total of 24 hours of fasting. Measurements will include oxygen expenditure expressed as KCal/min and measurements of key parameters in maintaining physiologic homeostasis, i.e. serum T4, T3, rT3, insulin; plasma glucose, norepinephrine and epinephrine. Individual response in relation to baseline will be measured.